I. Field of the Invention
The present invention relates to electrical discharge machining (EDM), and more particularly to an electrode tool in the form of a wire.
II. Description of the Prior Art
EDM methods involve the machining of a conductive workpiece through vaporization of a very small portion of the workpiece by a high frequency electric spark between the workpiece and an electrical discharge tool. One known EDM method involves a tool configured as a conductive metallic wire. The workpiece and the wire are displaced relative to one another, the wire for example being held in a particular location, and the workpiece disposed on a movable carriage and moved with respect to the wire. The characteristics of apparatus for performing such methods are well known, for example, as disclosed in U.S. Pat. No. 4,287,404 at column 1, lines 11 through 22, and U.S. Pat. No. 4,341,939 at column 1, lines 7 through 37.
It is also known to provide an oxidized surface on an EDM electrode wire by heating the wire in an oxidizing atmosphere prior to contacting the workpiece. However, this lessens the diameter of the conductive metal interior portion of the wire, and thereby lessens the current which can be carried by the wire without breakage or melting. The traction or pull which can be exerted on the wire tool is thus decreased. Since the machining speed of an EDM apparatus is dependent upon the current which can be carried by the electrode and the traction experienced by the electrode, such oxidizing treatment has not increased the machining speed to the degree expected.
Another attempt to increase the machining speed is disclosed in U.S. Pat. No. 4,287,404 and employs a wire tool having a metallic wire core of high mechanical strength (for example, steel) and a second metallic coating on the core, the coating having a low vaporization temperature. The second metallic coating is intended to protect the core against thermal shock from the electrical discharges, and thereby allow an increase in the power or frequency of the discharges. Such a wire electrode is disadvantageous, however, in that it readily short circuits with the workpiece, particularly when the metallic coating is thin. For example, the surface craters created by electrical discharges commonly employed in EDM apparatus are on the order of five micrometers in size, the same as the typical thickness of the metallic coating on the electrode wire. Moreover, the thin second metallic coating may have a poor electrical conductivity, which will also interfere with optimal operation of the apparatus. Nor is the tensile strength of the wire completely satisfactory. While the wire can be extruded through a die for sizing, the patent neither discloses nor suggests cold-drawing the coated wire an amount sufficient to improve its tensile strength while simultaneously decreasing the thickness of the metallic coating.
The invention disclosed in U.S. Pat. No. 4,341,939 attempted to solve these drawbacks by combining features of the two wire electrodes mentioned above. In that patent, the upper portion of a metallic coating on a wire core is oxidized by heating the wire at a temperature of about at least 600.degree. C. One effect observed was that the core metal and coating metal diffuse into each other upon such heating, to form an alloy layer covered by a much thinner film of an oxide of the coating metal. For example, when the core metal is copper and the coating metal is zinc, the resulting wire includes a copper core covered by a layer of brass, in turn covered by zinc oxide. While the electrode wire disclosed in this patent has been found very useful in practice, its use occasionally still encounters some of the attendant drawbacks of each of the metal and oxide coatings applied in prior methods and is thus not always the optimal construction for EDM tool wires.